1 /*

     2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     3  *

     4  * This code is free software; you can redistribute it and/or modify it

     5  * under the terms of the GNU General Public License version 2 only, as

     6  * published by the Free Software Foundation.  Sun designates this

     7  * particular file as subject to the "Classpath" exception as provided

     8  * by Sun in the LICENSE file that accompanied this code.

     9  *

    10  * This code is distributed in the hope that it will be useful, but WITHOUT

    11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    13  * version 2 for more details (a copy is included in the LICENSE file that

    14  * accompanied this code).

    15  *

    16  * You should have received a copy of the GNU General Public License version

    17  * 2 along with this work; if not, write to the Free Software Foundation,

    18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    19  *

    20  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    21  * CA 95054 USA or visit www.sun.com if you need additional information or

    22  * have any questions.

    23  */

    24 

    25 /*

    26  * This file is available under and governed by the GNU General Public

    27  * License version 2 only, as published by the Free Software Foundation.

    28  * However, the following notice accompanied the original version of this

    29  * file:

    30  *

    31  * Written by Doug Lea with assistance from members of JCP JSR-166

    32  * Expert Group and released to the public domain, as explained at

    33  * http://creativecommons.org/licenses/publicdomain

    34  */

    35 

    36 package java.util.concurrent;

    37 import java.util.concurrent.locks.*;

    38 import java.util.*;

    39 

    40 /**

    41  * A bounded {@linkplain BlockingQueue blocking queue} backed by an

    42  * array.  This queue orders elements FIFO (first-in-first-out).  The

    43  * <em>head</em> of the queue is that element that has been on the

    44  * queue the longest time.  The <em>tail</em> of the queue is that

    45  * element that has been on the queue the shortest time. New elements

    46  * are inserted at the tail of the queue, and the queue retrieval

    47  * operations obtain elements at the head of the queue.

    48  *

    49  * <p>This is a classic &quot;bounded buffer&quot;, in which a

    50  * fixed-sized array holds elements inserted by producers and

    51  * extracted by consumers.  Once created, the capacity cannot be

    52  * increased.  Attempts to <tt>put</tt> an element into a full queue

    53  * will result in the operation blocking; attempts to <tt>take</tt> an

    54  * element from an empty queue will similarly block.

    55  *

    56  * <p> This class supports an optional fairness policy for ordering

    57  * waiting producer and consumer threads.  By default, this ordering

    58  * is not guaranteed. However, a queue constructed with fairness set

    59  * to <tt>true</tt> grants threads access in FIFO order. Fairness

    60  * generally decreases throughput but reduces variability and avoids

    61  * starvation.

    62  *

    63  * <p>This class and its iterator implement all of the

    64  * <em>optional</em> methods of the {@link Collection} and {@link

    65  * Iterator} interfaces.

    66  *

    67  * <p>This class is a member of the

    68  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

    69  * Java Collections Framework</a>.

    70  *

    71  * @since 1.5

    72  * @author Doug Lea

    73  * @param <E> the type of elements held in this collection

    74  */

    75 public class ArrayBlockingQueue<E> extends AbstractQueue<E>

    76         implements BlockingQueue<E>, java.io.Serializable {

    77 

    78     /**

    79      * Serialization ID. This class relies on default serialization

    80      * even for the items array, which is default-serialized, even if

    81      * it is empty. Otherwise it could not be declared final, which is

    82      * necessary here.

    83      */

    84     private static final long serialVersionUID = -817911632652898426L;

    85 

    86     /** The queued items  */

    87     private final E[] items;

    88     /** items index for next take, poll or remove */

    89     private int takeIndex;

    90     /** items index for next put, offer, or add. */

    91     private int putIndex;

    92     /** Number of items in the queue */

    93     private int count;

    94 

    95     /*

    96      * Concurrency control uses the classic two-condition algorithm

    97      * found in any textbook.

    98      */

    99 

   100     /** Main lock guarding all access */

   101     private final ReentrantLock lock;

   102     /** Condition for waiting takes */

   103     private final Condition notEmpty;

   104     /** Condition for waiting puts */

   105     private final Condition notFull;

   106 

   107     // Internal helper methods

   108 

   109     /**

   110      * Circularly increment i.

   111      */

   112     final int inc(int i) {

   113         return (++i == items.length)? 0 : i;

   114     }

   115 

   116     /**

   117      * Inserts element at current put position, advances, and signals.

   118      * Call only when holding lock.

   119      */

   120     private void insert(E x) {

   121         items[putIndex] = x;

   122         putIndex = inc(putIndex);

   123         ++count;

   124         notEmpty.signal();

   125     }

   126 

   127     /**

   128      * Extracts element at current take position, advances, and signals.

   129      * Call only when holding lock.

   130      */

   131     private E extract() {

   132         final E[] items = this.items;

   133         E x = items[takeIndex];

   134         items[takeIndex] = null;

   135         takeIndex = inc(takeIndex);

   136         --count;

   137         notFull.signal();

   138         return x;

   139     }

   140 

   141     /**

   142      * Utility for remove and iterator.remove: Delete item at position i.

   143      * Call only when holding lock.

   144      */

   145     void removeAt(int i) {

   146         final E[] items = this.items;

   147         // if removing front item, just advance

   148         if (i == takeIndex) {

   149             items[takeIndex] = null;

   150             takeIndex = inc(takeIndex);

   151         } else {

   152             // slide over all others up through putIndex.

   153             for (;;) {

   154                 int nexti = inc(i);

   155                 if (nexti != putIndex) {

   156                     items[i] = items[nexti];

   157                     i = nexti;

   158                 } else {

   159                     items[i] = null;

   160                     putIndex = i;

   161                     break;

   162                 }

   163             }

   164         }

   165         --count;

   166         notFull.signal();

   167     }

   168 

   169     /**

   170      * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)

   171      * capacity and default access policy.

   172      *

   173      * @param capacity the capacity of this queue

   174      * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1

   175      */

   176     public ArrayBlockingQueue(int capacity) {

   177         this(capacity, false);

   178     }

   179 

   180     /**

   181      * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)

   182      * capacity and the specified access policy.

   183      *

   184      * @param capacity the capacity of this queue

   185      * @param fair if <tt>true</tt> then queue accesses for threads blocked

   186      *        on insertion or removal, are processed in FIFO order;

   187      *        if <tt>false</tt> the access order is unspecified.

   188      * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1

   189      */

   190     public ArrayBlockingQueue(int capacity, boolean fair) {

   191         if (capacity <= 0)

   192             throw new IllegalArgumentException();

   193         this.items = (E[]) new Object[capacity];

   194         lock = new ReentrantLock(fair);

   195         notEmpty = lock.newCondition();

   196         notFull =  lock.newCondition();

   197     }

   198 

   199     /**

   200      * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)

   201      * capacity, the specified access policy and initially containing the

   202      * elements of the given collection,

   203      * added in traversal order of the collection's iterator.

   204      *

   205      * @param capacity the capacity of this queue

   206      * @param fair if <tt>true</tt> then queue accesses for threads blocked

   207      *        on insertion or removal, are processed in FIFO order;

   208      *        if <tt>false</tt> the access order is unspecified.

   209      * @param c the collection of elements to initially contain

   210      * @throws IllegalArgumentException if <tt>capacity</tt> is less than

   211      *         <tt>c.size()</tt>, or less than 1.

   212      * @throws NullPointerException if the specified collection or any

   213      *         of its elements are null

   214      */

   215     public ArrayBlockingQueue(int capacity, boolean fair,

   216                               Collection<? extends E> c) {

   217         this(capacity, fair);

   218         if (capacity < c.size())

   219             throw new IllegalArgumentException();

   220 

   221         for (Iterator<? extends E> it = c.iterator(); it.hasNext();)

   222             add(it.next());

   223     }

   224 

   225     /**

   226      * Inserts the specified element at the tail of this queue if it is

   227      * possible to do so immediately without exceeding the queue's capacity,

   228      * returning <tt>true</tt> upon success and throwing an

   229      * <tt>IllegalStateException</tt> if this queue is full.

   230      *

   231      * @param e the element to add

   232      * @return <tt>true</tt> (as specified by {@link Collection#add})

   233      * @throws IllegalStateException if this queue is full

   234      * @throws NullPointerException if the specified element is null

   235      */

   236     public boolean add(E e) {

   237         return super.add(e);

   238     }

   239 

   240     /**

   241      * Inserts the specified element at the tail of this queue if it is

   242      * possible to do so immediately without exceeding the queue's capacity,

   243      * returning <tt>true</tt> upon success and <tt>false</tt> if this queue

   244      * is full.  This method is generally preferable to method {@link #add},

   245      * which can fail to insert an element only by throwing an exception.

   246      *

   247      * @throws NullPointerException if the specified element is null

   248      */

   249     public boolean offer(E e) {

   250         if (e == null) throw new NullPointerException();

   251         final ReentrantLock lock = this.lock;

   252         lock.lock();

   253         try {

   254             if (count == items.length)

   255                 return false;

   256             else {

   257                 insert(e);

   258                 return true;

   259             }

   260         } finally {

   261             lock.unlock();

   262         }

   263     }

   264 

   265     /**

   266      * Inserts the specified element at the tail of this queue, waiting

   267      * for space to become available if the queue is full.

   268      *

   269      * @throws InterruptedException {@inheritDoc}

   270      * @throws NullPointerException {@inheritDoc}

   271      */

   272     public void put(E e) throws InterruptedException {

   273         if (e == null) throw new NullPointerException();

   274         final E[] items = this.items;

   275         final ReentrantLock lock = this.lock;

   276         lock.lockInterruptibly();

   277         try {

   278             try {

   279                 while (count == items.length)

   280                     notFull.await();

   281             } catch (InterruptedException ie) {

   282                 notFull.signal(); // propagate to non-interrupted thread

   283                 throw ie;

   284             }

   285             insert(e);

   286         } finally {

   287             lock.unlock();

   288         }

   289     }

   290 

   291     /**

   292      * Inserts the specified element at the tail of this queue, waiting

   293      * up to the specified wait time for space to become available if

   294      * the queue is full.

   295      *

   296      * @throws InterruptedException {@inheritDoc}

   297      * @throws NullPointerException {@inheritDoc}

   298      */

   299     public boolean offer(E e, long timeout, TimeUnit unit)

   300         throws InterruptedException {

   301 

   302         if (e == null) throw new NullPointerException();

   303         long nanos = unit.toNanos(timeout);

   304         final ReentrantLock lock = this.lock;

   305         lock.lockInterruptibly();

   306         try {

   307             for (;;) {

   308                 if (count != items.length) {

   309                     insert(e);

   310                     return true;

   311                 }

   312                 if (nanos <= 0)

   313                     return false;

   314                 try {

   315                     nanos = notFull.awaitNanos(nanos);

   316                 } catch (InterruptedException ie) {

   317                     notFull.signal(); // propagate to non-interrupted thread

   318                     throw ie;

   319                 }

   320             }

   321         } finally {

   322             lock.unlock();

   323         }

   324     }

   325 

   326     public E poll() {

   327         final ReentrantLock lock = this.lock;

   328         lock.lock();

   329         try {

   330             if (count == 0)

   331                 return null;

   332             E x = extract();

   333             return x;

   334         } finally {

   335             lock.unlock();

   336         }

   337     }

   338 

   339     public E take() throws InterruptedException {

   340         final ReentrantLock lock = this.lock;

   341         lock.lockInterruptibly();

   342         try {

   343             try {

   344                 while (count == 0)

   345                     notEmpty.await();

   346             } catch (InterruptedException ie) {

   347                 notEmpty.signal(); // propagate to non-interrupted thread

   348                 throw ie;

   349             }

   350             E x = extract();

   351             return x;

   352         } finally {

   353             lock.unlock();

   354         }

   355     }

   356 

   357     public E poll(long timeout, TimeUnit unit) throws InterruptedException {

   358         long nanos = unit.toNanos(timeout);

   359         final ReentrantLock lock = this.lock;

   360         lock.lockInterruptibly();

   361         try {

   362             for (;;) {

   363                 if (count != 0) {

   364                     E x = extract();

   365                     return x;

   366                 }

   367                 if (nanos <= 0)

   368                     return null;

   369                 try {

   370                     nanos = notEmpty.awaitNanos(nanos);

   371                 } catch (InterruptedException ie) {

   372                     notEmpty.signal(); // propagate to non-interrupted thread

   373                     throw ie;

   374                 }

   375 

   376             }

   377         } finally {

   378             lock.unlock();

   379         }

   380     }

   381 

   382     public E peek() {

   383         final ReentrantLock lock = this.lock;

   384         lock.lock();

   385         try {

   386             return (count == 0) ? null : items[takeIndex];

   387         } finally {

   388             lock.unlock();

   389         }

   390     }

   391 

   392     // this doc comment is overridden to remove the reference to collections

   393     // greater in size than Integer.MAX_VALUE

   394     /**

   395      * Returns the number of elements in this queue.

   396      *

   397      * @return the number of elements in this queue

   398      */

   399     public int size() {

   400         final ReentrantLock lock = this.lock;

   401         lock.lock();

   402         try {

   403             return count;

   404         } finally {

   405             lock.unlock();

   406         }

   407     }

   408 

   409     // this doc comment is a modified copy of the inherited doc comment,

   410     // without the reference to unlimited queues.

   411     /**

   412      * Returns the number of additional elements that this queue can ideally

   413      * (in the absence of memory or resource constraints) accept without

   414      * blocking. This is always equal to the initial capacity of this queue

   415      * less the current <tt>size</tt> of this queue.

   416      *

   417      * <p>Note that you <em>cannot</em> always tell if an attempt to insert

   418      * an element will succeed by inspecting <tt>remainingCapacity</tt>

   419      * because it may be the case that another thread is about to

   420      * insert or remove an element.

   421      */

   422     public int remainingCapacity() {

   423         final ReentrantLock lock = this.lock;

   424         lock.lock();

   425         try {

   426             return items.length - count;

   427         } finally {

   428             lock.unlock();

   429         }

   430     }

   431 

   432     /**

   433      * Removes a single instance of the specified element from this queue,

   434      * if it is present.  More formally, removes an element <tt>e</tt> such

   435      * that <tt>o.equals(e)</tt>, if this queue contains one or more such

   436      * elements.

   437      * Returns <tt>true</tt> if this queue contained the specified element

   438      * (or equivalently, if this queue changed as a result of the call).

   439      *

   440      * @param o element to be removed from this queue, if present

   441      * @return <tt>true</tt> if this queue changed as a result of the call

   442      */

   443     public boolean remove(Object o) {

   444         if (o == null) return false;

   445         final E[] items = this.items;

   446         final ReentrantLock lock = this.lock;

   447         lock.lock();

   448         try {

   449             int i = takeIndex;

   450             int k = 0;

   451             for (;;) {

   452                 if (k++ >= count)

   453                     return false;

   454                 if (o.equals(items[i])) {

   455                     removeAt(i);

   456                     return true;

   457                 }

   458                 i = inc(i);

   459             }

   460 

   461         } finally {

   462             lock.unlock();

   463         }

   464     }

   465 

   466     /**

   467      * Returns <tt>true</tt> if this queue contains the specified element.

   468      * More formally, returns <tt>true</tt> if and only if this queue contains

   469      * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.

   470      *

   471      * @param o object to be checked for containment in this queue

   472      * @return <tt>true</tt> if this queue contains the specified element

   473      */

   474     public boolean contains(Object o) {

   475         if (o == null) return false;

   476         final E[] items = this.items;

   477         final ReentrantLock lock = this.lock;

   478         lock.lock();

   479         try {

   480             int i = takeIndex;

   481             int k = 0;

   482             while (k++ < count) {

   483                 if (o.equals(items[i]))

   484                     return true;

   485                 i = inc(i);

   486             }

   487             return false;

   488         } finally {

   489             lock.unlock();

   490         }

   491     }

   492 

   493     /**

   494      * Returns an array containing all of the elements in this queue, in

   495      * proper sequence.

   496      *

   497      * <p>The returned array will be "safe" in that no references to it are

   498      * maintained by this queue.  (In other words, this method must allocate

   499      * a new array).  The caller is thus free to modify the returned array.

   500      *

   501      * <p>This method acts as bridge between array-based and collection-based

   502      * APIs.

   503      *

   504      * @return an array containing all of the elements in this queue

   505      */

   506     public Object[] toArray() {

   507         final E[] items = this.items;

   508         final ReentrantLock lock = this.lock;

   509         lock.lock();

   510         try {

   511             Object[] a = new Object[count];

   512             int k = 0;

   513             int i = takeIndex;

   514             while (k < count) {

   515                 a[k++] = items[i];

   516                 i = inc(i);

   517             }

   518             return a;

   519         } finally {

   520             lock.unlock();

   521         }

   522     }

   523 

   524     /**

   525      * Returns an array containing all of the elements in this queue, in

   526      * proper sequence; the runtime type of the returned array is that of

   527      * the specified array.  If the queue fits in the specified array, it

   528      * is returned therein.  Otherwise, a new array is allocated with the

   529      * runtime type of the specified array and the size of this queue.

   530      *

   531      * <p>If this queue fits in the specified array with room to spare

   532      * (i.e., the array has more elements than this queue), the element in

   533      * the array immediately following the end of the queue is set to

   534      * <tt>null</tt>.

   535      *

   536      * <p>Like the {@link #toArray()} method, this method acts as bridge between

   537      * array-based and collection-based APIs.  Further, this method allows

   538      * precise control over the runtime type of the output array, and may,

   539      * under certain circumstances, be used to save allocation costs.

   540      *

   541      * <p>Suppose <tt>x</tt> is a queue known to contain only strings.

   542      * The following code can be used to dump the queue into a newly

   543      * allocated array of <tt>String</tt>:

   544      *

   545      * <pre>

   546      *     String[] y = x.toArray(new String[0]);</pre>

   547      *

   548      * Note that <tt>toArray(new Object[0])</tt> is identical in function to

   549      * <tt>toArray()</tt>.

   550      *

   551      * @param a the array into which the elements of the queue are to

   552      *          be stored, if it is big enough; otherwise, a new array of the

   553      *          same runtime type is allocated for this purpose

   554      * @return an array containing all of the elements in this queue

   555      * @throws ArrayStoreException if the runtime type of the specified array

   556      *         is not a supertype of the runtime type of every element in

   557      *         this queue

   558      * @throws NullPointerException if the specified array is null

   559      */

   560     public <T> T[] toArray(T[] a) {

   561         final E[] items = this.items;

   562         final ReentrantLock lock = this.lock;

   563         lock.lock();

   564         try {

   565             if (a.length < count)

   566                 a = (T[])java.lang.reflect.Array.newInstance(

   567                     a.getClass().getComponentType(),

   568                     count

   569                     );

   570 

   571             int k = 0;

   572             int i = takeIndex;

   573             while (k < count) {

   574                 a[k++] = (T)items[i];

   575                 i = inc(i);

   576             }

   577             if (a.length > count)

   578                 a[count] = null;

   579             return a;

   580         } finally {

   581             lock.unlock();

   582         }

   583     }

   584 

   585     public String toString() {

   586         final ReentrantLock lock = this.lock;

   587         lock.lock();

   588         try {

   589             return super.toString();

   590         } finally {

   591             lock.unlock();

   592         }

   593     }

   594 

   595     /**

   596      * Atomically removes all of the elements from this queue.

   597      * The queue will be empty after this call returns.

   598      */

   599     public void clear() {

   600         final E[] items = this.items;

   601         final ReentrantLock lock = this.lock;

   602         lock.lock();

   603         try {

   604             int i = takeIndex;

   605             int k = count;

   606             while (k-- > 0) {

   607                 items[i] = null;

   608                 i = inc(i);

   609             }

   610             count = 0;

   611             putIndex = 0;

   612             takeIndex = 0;

   613             notFull.signalAll();

   614         } finally {

   615             lock.unlock();

   616         }

   617     }

   618 

   619     /**

   620      * @throws UnsupportedOperationException {@inheritDoc}

   621      * @throws ClassCastException            {@inheritDoc}

   622      * @throws NullPointerException          {@inheritDoc}

   623      * @throws IllegalArgumentException      {@inheritDoc}

   624      */

   625     public int drainTo(Collection<? super E> c) {

   626         if (c == null)

   627             throw new NullPointerException();

   628         if (c == this)

   629             throw new IllegalArgumentException();

   630         final E[] items = this.items;

   631         final ReentrantLock lock = this.lock;

   632         lock.lock();

   633         try {

   634             int i = takeIndex;

   635             int n = 0;

   636             int max = count;

   637             while (n < max) {

   638                 c.add(items[i]);

   639                 items[i] = null;

   640                 i = inc(i);

   641                 ++n;

   642             }

   643             if (n > 0) {

   644                 count = 0;

   645                 putIndex = 0;

   646                 takeIndex = 0;

   647                 notFull.signalAll();

   648             }

   649             return n;

   650         } finally {

   651             lock.unlock();

   652         }

   653     }

   654 

   655     /**

   656      * @throws UnsupportedOperationException {@inheritDoc}

   657      * @throws ClassCastException            {@inheritDoc}

   658      * @throws NullPointerException          {@inheritDoc}

   659      * @throws IllegalArgumentException      {@inheritDoc}

   660      */

   661     public int drainTo(Collection<? super E> c, int maxElements) {

   662         if (c == null)

   663             throw new NullPointerException();

   664         if (c == this)

   665             throw new IllegalArgumentException();

   666         if (maxElements <= 0)

   667             return 0;

   668         final E[] items = this.items;

   669         final ReentrantLock lock = this.lock;

   670         lock.lock();

   671         try {

   672             int i = takeIndex;

   673             int n = 0;

   674             int sz = count;

   675             int max = (maxElements < count)? maxElements : count;

   676             while (n < max) {

   677                 c.add(items[i]);

   678                 items[i] = null;

   679                 i = inc(i);

   680                 ++n;

   681             }

   682             if (n > 0) {

   683                 count -= n;

   684                 takeIndex = i;

   685                 notFull.signalAll();

   686             }

   687             return n;

   688         } finally {

   689             lock.unlock();

   690         }

   691     }

   692 

   693 

   694     /**

   695      * Returns an iterator over the elements in this queue in proper sequence.

   696      * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that

   697      * will never throw {@link ConcurrentModificationException},

   698      * and guarantees to traverse elements as they existed upon

   699      * construction of the iterator, and may (but is not guaranteed to)

   700      * reflect any modifications subsequent to construction.

   701      *

   702      * @return an iterator over the elements in this queue in proper sequence

   703      */

   704     public Iterator<E> iterator() {

   705         final ReentrantLock lock = this.lock;

   706         lock.lock();

   707         try {

   708             return new Itr();

   709         } finally {

   710             lock.unlock();

   711         }

   712     }

   713 

   714     /**

   715      * Iterator for ArrayBlockingQueue

   716      */

   717     private class Itr implements Iterator<E> {

   718         /**

   719          * Index of element to be returned by next,

   720          * or a negative number if no such.

   721          */

   722         private int nextIndex;

   723 

   724         /**

   725          * nextItem holds on to item fields because once we claim

   726          * that an element exists in hasNext(), we must return it in

   727          * the following next() call even if it was in the process of

   728          * being removed when hasNext() was called.

   729          */

   730         private E nextItem;

   731 

   732         /**

   733          * Index of element returned by most recent call to next.

   734          * Reset to -1 if this element is deleted by a call to remove.

   735          */

   736         private int lastRet;

   737 

   738         Itr() {

   739             lastRet = -1;

   740             if (count == 0)

   741                 nextIndex = -1;

   742             else {

   743                 nextIndex = takeIndex;

   744                 nextItem = items[takeIndex];

   745             }

   746         }

   747 

   748         public boolean hasNext() {

   749             /*

   750              * No sync. We can return true by mistake here

   751              * only if this iterator passed across threads,

   752              * which we don't support anyway.

   753              */

   754             return nextIndex >= 0;

   755         }

   756 

   757         /**

   758          * Checks whether nextIndex is valid; if so setting nextItem.

   759          * Stops iterator when either hits putIndex or sees null item.

   760          */

   761         private void checkNext() {

   762             if (nextIndex == putIndex) {

   763                 nextIndex = -1;

   764                 nextItem = null;

   765             } else {

   766                 nextItem = items[nextIndex];

   767                 if (nextItem == null)

   768                     nextIndex = -1;

   769             }

   770         }

   771 

   772         public E next() {

   773             final ReentrantLock lock = ArrayBlockingQueue.this.lock;

   774             lock.lock();

   775             try {

   776                 if (nextIndex < 0)

   777                     throw new NoSuchElementException();

   778                 lastRet = nextIndex;

   779                 E x = nextItem;

   780                 nextIndex = inc(nextIndex);

   781                 checkNext();

   782                 return x;

   783             } finally {

   784                 lock.unlock();

   785             }

   786         }

   787 

   788         public void remove() {

   789             final ReentrantLock lock = ArrayBlockingQueue.this.lock;

   790             lock.lock();

   791             try {

   792                 int i = lastRet;

   793                 if (i == -1)

   794                     throw new IllegalStateException();

   795                 lastRet = -1;

   796 

   797                 int ti = takeIndex;

   798                 removeAt(i);

   799                 // back up cursor (reset to front if was first element)

   800                 nextIndex = (i == ti) ? takeIndex : i;

   801                 checkNext();

   802             } finally {

   803                 lock.unlock();

   804             }

   805         }

   806     }

   807 }